Mingtan dam and power station is a reservoir below Sun Moon Lake. The two reservoirs work together as a pump-storage power station. This is a technology we use in the states as well.
When the demand on the power grid is high water is released from the upper reservoir to turn the turbines and create power. Usually overnight, when demand on the power grid is low, water is pumped back to the upper reservoir and stored for its next use.
The Taiwan Power Company gave us a good presentation on their facility. A scenic and productive location.